Allow typing-extensions version 4.x (ezyang#107)

Currently, `[tool.poetry.dependencies]` is configured to only allow `typing-extensions` with major version `3`. `typing-extensions` was bumped to version `4`; I believe the only significant change is dropping Python 3.5 support, which is already unsupported by other dependencies. Allow version 4 to be used to avoid unnecessary package conflicts when installed in existing virtualenvs.

Author: amartani

poetry.lock

@@ -22,7 +22,7 @@ dataclasses = { version = "^0.8", python = "<3.7" }
 importlib-metadata = "^3"
 python = "^3.6"
 requests = "^2"
-typing-extensions = "^3"
+typing-extensions = ">=3 <5"
 
 [tool.poetry.dev-dependencies]
 expecttest = "^0.1"